The present invention relates generally to the field of circulating fluidized bed (CFB) reactors or boilers such as those used in electric power generation facilities and, in particular, to a new and useful CFB reactor arrangement which permits temperature control within the CFB reaction chamber and/or of the effluent solids. The CFB reactor arrangement according to the invention contains and supports not only the CFB but also one or more bubbling fluidized bed(s) (BFB""s) in a lower portion of the CFB reactor enclosure; i.e., one or more slow bubbling bed region(s) are maintained and located within a fast CFB region. An arrangement of heating surface is located within the bubbling fluidized bed(s) (BFB""s). Heat transfer to the heating surface is controlled by providing separately controlled fluidizing gas to the bubbling fluidized bed(s) (BFB""s) to either maintain a desired bed level or control a throughput of solids through the bubbling fluidized bed(s) (BFB""s).
Most prior arts bubbling bed heat exchangers known to the inventors are located outside of the CFB reaction chamber and occupy at least one of the enclosure walls.
For example, U.S. Pat. Nos. 5,526,775 and 5,533,471 to Hyppxc3xa4nen each disclose a CFB having an adjacent bubbling fluidized bed with an integral heat exchanger. U.S. Pat. No. 5,533,471 teaches placing the slow bubbling fluidized bed below and to the side of the bottom of the faster moving CFB chamber. In U.S. Pat. No. 5,526,775, the slow bubbling bed is above and to the side of the fast CFB. Each of the slow beds is controlled by permitting particles to escape back into the main CFB chamber from an opening in the side of the slow bed chamber. These heat exchangers further require a different gas distribution grid level for each bed, which substantially complicates the structure of the CFB systems. The plan area of the CFB can be increased as a result.
Other patents disclose heat exchanger elements located above the grid of a CFB furnace, but not within a slow bubbling bed region of a fast CFB. U.S. Pat. No. 5,190,451 to Goldbach, for example, illustrates a CFB chamber having a heat exchanger immersed within a fluidized bed at the lower end of the chamber. The bed has only one air injector for controlling the circulation rate for the entire bed.
U.S. Pat. No. 5,299,532 to Dietz discloses a CFB having a recycle chamber immediately adjacent the main CFB chamber. The recycle chamber receives partially combusted particulate from a cyclone separator connected between the recycle chamber and the upper exhaust of the main CFB chamber. A heat exchanger is provided inside the recycle chamber, and the recycle chamber is separated from the main CFB chamber by water walls and occupies part of the lower portion of the furnace enclosure; the recycle chamber does not extend outwardly from the furnace enclosure.
U.S. Pat. No. 5,184,671 to Alliston et al. teaches a heat exchanger having multiple fluidized bed regions. One region has heat exchange surfaces, while the other regions are used to control the rate of heat transfer between the fluidized bed material and the heat exchanger surfaces.
None of these prior art bubbling beds is incorporated in a manner which simplifies the overall construction of the CFB reactor and permits easy access to enclosure walls for feeding reagents, maintenance and inspections.
The present invention seeks to overcome the limitations of the prior art CFB slow bed heat exchangers by providing a CFB boiler or reactor having an internal heat exchanger in a slow bubbling bed, and without increasing the plan area of the CFB.
Accordingly, one aspect of the present invention is drawn to a circulating fluidized bed (CFB) boiler, comprising: a CFB reaction chamber having side walls and a grid defining a floor at a lower end of the CFB reaction chamber for providing fluidizing gas into the CFB reaction chamber. Means are provided for supplying an amount of fluidizing gas to a first portion of the grid sufficient to produce a fast moving bed of fluidized solids in a first zone of the CFB reaction chamber, and for providing an amount of fluidizing gas to a second portion of the grid sufficient to produce a bubbling fluidized bed of fluidized solids in a second zone of the CFB reaction chamber. The amount of fluidizing gas provided to one zone is controllable independently of the amount of fluidizing gas provided to the other zone. Finally, means are provided for removing solids from the first and second zones for purging the solids from or recycling the solids to the CFB boiler to control the fast moving bed.
Thus, the CFB boiler is partitioned into two portions: a first portion or zone which is operated as a fast moving circulating fluidized bed, and a second region or zone which is operated as a slow bubbling fluidized bed.
The slow bubbling bed height is controlled within the range corresponding to the height of its enclosure walls. Mechanisms for controlling the slow bed height include outlets through the top of the enclosure and a valved outlet through the bottom side edges of the enclosure.
In an alternate embodiment, a portion of the floor-level grid has openings sufficient to allow particles to fall through. A heat exchanger is located directly below the main CFB chamber. A secondary fluidizing gas supply is provided in the region of the grid above the heat exchanger. The amount of particles falling through into the area below the grid with the slow bubbling bed can be controlled by controlling their purge or recycle rate.
In a further embodiment, the above-grid enclosure for one heat exchanger is combined with the below-grid position of a second heat exchanger.
The improved CFB design of the invention permits a reduced footprint size of the CFB and allows the enclosure walls to be straightened. The design is simpler in construction and provides easier access to the enclosure walls for feeding reagents.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.